1. Field of the Invention
The present invention relates to a highly wear resistant protective coating for metallic, highly stressed surfaces or substrates, and more particularly to a protective coating which is comprised of two or more hard substances and has a total thickness ranging from 0.1 to 10 .mu..
2. Discussion of the Prior Art
Hard substance protective coatings in the form of single or multiple layers on steel or hard metal substrates produced by a chemical vapor deposition process (CVD) or a physical vapor deposition (PVD) process constitute a significant advance in improved wear resistance and, thus, in service life of cutting materials or parts that are subject to wear. A hard substance coating imparts wear protection to the tough substrate by increasing the abrasion resistance of its surface, by reducing friction and thus temperature, as well as by reducing diffusion and adhesion between the material and the workpiece or chip.
Such composite materials often are characterized, however, by insufficient adhesion between the substrate material and the coating, by insufficient toughness of the coating, and by lack of resistance to alternating stresses. Multiple-layer coatings have been provided in an attempt to solve these problems. Significant improvements compared to single-layer coatings have resulted, but the aforementioned insufficient characteristics of the substrate/coating system have not yet been completely eliminated.
Multilayered coatings of hard substances on hard metal substrates are discussed, for example, in the metallurgical journal Zeitschrift fur Metallkunde, Volume 75, No. 11, Nov. 1984, at pages 874-880. The publication mentions, for example, a ten-layer protective coating in which a hard metal substrate is coated in turn with a titanium carbide layer (TiC), a titanium carbonitride layer (Ti(C,N)) and a layer sequence of four intermediate layers and four ceramic layers based on Al.sub.2 O.sub.3. The publication also refers to a multilayered coating including layers of titanium carbide (TiC), titanium carbonitride (Ti(C,N)), and titanium nitride (TiN), and having a thickness totaling approximately 10 .mu.. For coating temperatures below 773.degree. K, the physical vapor deposition (PVD) method including reactive cathode sputtering at pressures of .ltoreq.10.sup.-2 bar of N.sub.2 or Ar, was found to be useful. Although a significant improvement compared to single-layer coatings as noted above, the aforementioned insufficient characteristics of the substrate/coating system have not yet been completely eliminated.